1. The Field of The Invention
The present invention relates to an improved, miniaturized electro-acoustic surface-wave filter device and in particular to an improved transducer configuration.
2. Description of the Prior Art
Conventional surface-wave filters employ two interdigital electrode transducer structures fabricated on the surface of a piezoelectric substrate. The detailed design of these transducers determines the frequency response of the filter. Sophisticated filter design usually involves varying the finger overlap within a transducer, a process termed apodization. Several examples of the prior art are briefly discussed below.
U.S. Pat. No. 3,360,749 describes an interdigital electrode structure, fabricated onto one surface of a piezoelectric substrate for the purpose of launching surface elastic waves.
U.S. Pat. No. 3,515,911 describes the design and fabrication of an acoustic surface-wave transducer whose desired impulse response is expressed as the convolution of two separate pulse sequences; one sequence is embodied in a stack of piezoelectric bulk wave transducer elements and passive pacers, the second is embodied in an arrangement of mechanical feet which are attached both to the bottom of the transducer stack and to the intended substrate.
U.S. Pat. No. 3,573,673 discloses several ways to reduce the magnitude of crosstalk between first and second transducing devices spaced on the same surface of a body of piezoelectric material. These include: grounding adjacent electrodes of the two transducers; the inclusion of slanted shield electrodes; a balanced drive for the transducers; and shielding channels on the backside of the body. An angled orientation with respect to the ends of the body is employed to reduce edge reflections. U.S. Pat. No. 3,582,838 relates to a surface-wave device having unapodized combs in which the number of electrodes determines the trap frequencies. This device is also inductively resonated.
U.S. Pat. No. 3,600,710 discloses the use of transducers connected in series for impedance scaling but does not discuss this technique applied to shaping the transducer frequency response.
U.S. Pat. No. 3,663,899 suggests a surface-wave electro-acoustic filter having a transducer formed by overlapping, uniformly spaced teeth which define the Fourier transform of the transfer function of the device when operated as a filter.
U.S. Pat. No. 3,699,364 shows the use of dummy electrodes for eliminating phase distortion in the transducer portion of an apodized surface-wave device.
U.S. Pat. No. 3,727,155 discloses the use of quarter-wavelength-spaced electrodes ("split fingers") to reduce reflections between acoustic surface-wave transducers.
U.S. Pat. No. 3,792,381 employs flanking electrode arrays spaced within one wavelength of, and in a phase reversal relationship with, the central array of an acoustic surface-wave transducer in order to achieve some control over the frequency response.
U.S. Pat. No. 3,801,935 arranges transducer electrode groups within a uniformly spaced electrode array such that they are electrically and acoustically in series, thereby, achieving impedance scaling and some control over the shape of the transducer frequency response.
U.S. Pat. No. 3,801,937 teaches the use of flanking electrode arrays in an unapodized acoustic surface-wave transducer, these flanking arrays being so positioned and phased with respect to a central array as to provide cosine-on-a-pedestal frequency response over a specified bandwidth as required for temporal sidelobe suppression in dispersive pulse compression filters.
U.S. Pat. No. 3,836,876 discloses the principle of the multistrip coupler, used as a track changing means in acoustic surface-wave devices to provide suppression of undesired bulk wave modes. Subsequently it has been employed to effectively combine the responses of two apodized transducers in a surface-wave device.
Those skilled in this art have generally been unable to design filters employing apodization of both transducers without the use of a multistrip coupler (MSC). Thus, the designer is ordinarily constrained to use to best advantage the relatively inflexible response of one unapodized transducer, or to more than double the filter area in order to make use of the multistrip coupler.
Substrate cost is a principal determinent in the final cost of a commercial surface-wave filter. Thus minimization of the electroded area is an important design goal. The size of an individual transducer is naturally governed by its length (measured perpendicular to the long axes of the interdigital electrodes) and width (measured parallel to the long axes of the interdigital electrodes). The length is largely predetermined by the specified filter frequency response, and is relatively inflexible. The width or aperture may be reduced, but diffraction of the launched acoustic beam sets a lower limit. If the width is reduced into the diffraction region, rather intractable phase and amplitude distortions degrade the filter response, particularly in critical stopband and/or trap regions. Clearly an unapodized transducer can be width-reduced further than one containing a great number of small-overlap fingers.
The important requirements of a television IF filter are a specified passband, with strong rejection of picture and sound carriers from adjacent channels. Meeting this specification with a surface-wave filter ordinarily requires either an apodized transducer with wide aperture, or else judicious placement of the deep, narrow null responses of an unapodized design.